1. Field of Invention
The present invention relates to a method of controlling an interconnected system that supplies power to a power system, and includes a power generator (e.g., wind power generator) that changes in output, in combination with an electric power storage-compensation device that includes a sodium-sulfur battery.
2. Description of Related Art
In recent years, a natural-energy power generator that generates power using wind, solar power, geothermal energy, or the like has attracted attention, and has been put to practical use. A natural-energy power generator is a clean power generator that utilizes an inexhaustible natural energy source instead of limited resources (e.g., petroleum), and can suppress carbon dioxide emissions. Therefore, companies, autonomies, and the like have increasingly employed a natural-energy power generator in view of prevention of global warming.
However, since the amount of natural energy obtained varies from hour to hour, a natural-energy power generator inevitably changes in output. This is an obstacle to widespread use of a natural-energy power generator. Therefore, when employing a natural-energy power generator, it is preferable to construct an interconnected (power generation) system by combining the natural-energy power generator with an electric power storage-compensation device that mainly includes a plurality of secondary batteries.
In particular, a sodium-sulfur battery among the other secondary batteries has a high energy density, achieves a high output within a short time, and exhibits a rapid response. Therefore, a sodium-sulfur battery may suitably be used to compensate for a change in output of a natural-energy power generator that may occur of the order of several hundred milliseconds to several seconds by providing a bidirectional converter that controls charging and discharging in combination with the sodium-sulfur battery. In other words, an interconnected system that includes a natural-energy power generator and an electric power storage-compensation device that includes a plurality of sodium-sulfur batteries is a desirable power generation system.
Since a natural-energy power generator changes in output, an electric power storage-compensation device frequently receives and outputs power. That is, a sodium-sulfur battery that is included in the electric power storage-compensation device is repeatedly charged and discharged. This makes it difficult to accurately manage the battery level of the sodium-sulfur battery, so that it may suddenly become impossible to charge or discharge the sodium-sulfur battery (e.g., the operation of the sodium-sulfur battery stops when compensating for a change in output of the natural-energy power generator). Various methods have been disclosed to control a sodium-sulfur battery that is included in an electric power storage-compensation device (see JP-A-2003-317808, for example).
The sodium-sulfur battery that is included in the electric power storage-compensation device of the interconnected system suppresses or eliminates a change in power generation schedule that is designated manually or using a computer or the like taking account of a change in natural energy power generation. The interconnected system plans a power generation schedule based on predicted natural-energy power generation and the battery level, and supplies power from the interconnected system to the power system according to the power generation schedule. When it is impossible to generate power using natural energy (e.g., no wind) for a long time, the planned power generation value is normally set to 0 kW (i.e., power is not supplied to the power system). In this case, however, it is necessary to supply power to the local load of the interconnected system. Therefore, the sodium-sulfur battery is discharged to supply power to the local load, so that the battery level decreases.
For example, when using an interconnected system 8 shown in FIG. 1 that includes a wind power generator 7 (natural-energy power generator), an electric power storage-compensation device 5, and a local load 11, in the case where the planned power generation value is 0 kW (see FIG. 4) (i.e., power PT (thick solid line in FIG. 4) measured by a wattmeter 48 is 0 kW), a sodium-sulfur battery 3 is charged when power PA+PC (broken line in FIG. 4) has exceeded 0 kW. On the other hand, when the power PA+PC is less than 0 kW, the sodium-sulfur battery 3 is discharged to compensate for lack of power, so that the battery level decreases.
In this case, the amount of power to be discharged can be reduced (i.e., a decrease in battery level can be suppressed) by setting the planned power generation value to a value at which power is supplied to the interconnected system from the power system 1 (see FIG. 5). In this case, the interconnected system 8 is charged from the power system 1. In other words, the interconnected system 8 which should charge power to the power system 1 is supplied power from the power system 1. This situation is not preferred.
When the planned power generation value is set to a value at which power is supplied to the interconnected system from the power system 1 (i.e., power PT is set to be a negative value), the sodium-sulfur battery is charged when the power PA+PC has exceeded the planned power generation value. However, when the power PA+PC has exceeded the planned power generation value, but is less than 0 kW, the sodium-sulfur battery 3 is charged from the power system 1. On the other hand, when the power PA+PC is less than the planned power generation value, the sodium-sulfur battery 3 is discharged, so that the battery level decreases.
As shown in FIG. 6, power generated using natural energy and the local load power may be monitored, and a situation in which the sodium-sulfur battery 3 is charged from the power system may be prevented by changing the power generation schedule. However, this increases burden on the operator.
The present invention was conceived in view of the above problems. An object of the present invention is to provide a method of controlling an interconnected system that can suppress a decrease in battery level of a sodium-sulfur battery when it is impossible to generate power using natural energy for a long time.